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From: Deepak Gupta <debug@rivosinc.com>
Date: Wed, 14 Feb 2024 20:08:22 -0800
Message-ID: <CAKC1njQk7LajEMffAVMb76j6ndrixWyRgHf8XudSL-AzNn4s_g@mail.gmail.com>
Subject: Re: [PATCH RFT v5 1/7] Documentation: userspace-api: Add shadow stack
 API documentation
To: Mark Brown <broonie@kernel.org>
Cc: "Rick P. Edgecombe" <rick.p.edgecombe@intel.com>, Szabolcs Nagy <Szabolcs.Nagy@arm.com>, 
	"H.J. Lu" <hjl.tools@gmail.com>, Florian Weimer <fweimer@redhat.com>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>, 
	Vincent Guittot <vincent.guittot@linaro.org>, Dietmar Eggemann <dietmar.eggemann@arm.com>, 
	Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>, 
	Daniel Bristot de Oliveira <bristot@redhat.com>, Valentin Schneider <vschneid@redhat.com>, 
	Christian Brauner <brauner@kernel.org>, Shuah Khan <shuah@kernel.org>, linux-kernel@vger.kernel.org, 
	Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, 
	Kees Cook <keescook@chromium.org>, jannh@google.com, linux-kselftest@vger.kernel.org, 
	linux-api@vger.kernel.org
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On Fri, Feb 2, 2024 at 4:05=E2=80=AFPM Mark Brown <broonie@kernel.org> wrot=
e:
>
> There are a number of architectures with shadow stack features which we a=
re
> presenting to userspace with as consistent an API as we can (though there
> are some architecture specifics). Especially given that there are some
> important considerations for userspace code interacting directly with the
> feature let's provide some documentation covering the common aspects.
>
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  Documentation/userspace-api/index.rst        |  1 +
>  Documentation/userspace-api/shadow_stack.rst | 41 ++++++++++++++++++++++=
++++++
>  2 files changed, 42 insertions(+)
>
> diff --git a/Documentation/userspace-api/index.rst b/Documentation/usersp=
ace-api/index.rst
> index 09f61bd2ac2e..c142183d9c98 100644
> --- a/Documentation/userspace-api/index.rst
> +++ b/Documentation/userspace-api/index.rst
> @@ -27,6 +27,7 @@ place where this information is gathered.
>     iommufd
>     media/index
>     netlink/index
> +   shadow_stack
>     sysfs-platform_profile
>     vduse
>     futex2
> diff --git a/Documentation/userspace-api/shadow_stack.rst b/Documentation=
/userspace-api/shadow_stack.rst
> new file mode 100644
> index 000000000000..c6e5ab795b60
> --- /dev/null
> +++ b/Documentation/userspace-api/shadow_stack.rst
> @@ -0,0 +1,41 @@
> +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> +Shadow Stacks
> +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> +
> +Introduction
> +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> +
> +Several architectures have features which provide backward edge
> +control flow protection through a hardware maintained stack, only
> +writeable by userspace through very limited operations.  This feature
> +is referred to as shadow stacks on Linux, on x86 it is part of Intel
> +Control Enforcement Technology (CET), on arm64 it is Guarded Control
> +Stacks feature (FEAT_GCS) and for RISC-V it is the Zicfiss extension.
> +It is expected that this feature will normally be managed by the
> +system dynamic linker and libc in ways broadly transparent to
> +application code, this document covers interfaces and considerations
> +
> +
> +Enabling
> +=3D=3D=3D=3D=3D=3D=3D=3D
> +
> +Shadow stacks default to disabled when a userspace process is
> +executed, they can be enabled for the current thread with a syscall:
> +
> + - For x86 the ARCH_SHSTK_ENABLE arch_prctl()
> +
> +It is expected that this will normally be done by the dynamic linker.
> +Any new threads created by a thread with shadow stacks enabled will
> +themsleves have shadow stacks enabled.
> +
> +
> +Enablement considerations
> +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D
> +
> +- Returning from the function that enables shadow stacks without first
> +  disabling them will cause a shadow stack exception.  This includes
> +  any syscall wrapper or other library functions, the syscall will need
> +  to be inlined.
> +- A lock feature allows userspace to prevent disabling of shadow stacks.
> +- This that change the stack context like longjmp() or use of ucontext
> +  changes on signal return will need support from libc.

For enabling considerations, it will be good to have some words on ss token=
s
too, probably along the following lines.

- Shadow stack tokens: During shadow stack switches (either by user mode
or kernel), to prevent inadvertent shadow stack pivoting, it is expected to=
 save
some predefined formatted token during shadow stack save operation and
validating the token during shadow stack restore operation.

>
> --
> 2.30.2
>